When you deposit you receive an individual deposit address, that deposit along with all the others, then needs to be swept into a central wallet to be stored. That needs another transaction fee to be paid. It can be argued that charging a deposit fee is a fairer way to spread the cost than just charging extra on withdrawals to cover these sweeping fee costs as many other sites do. So this way, for example, someone who makes five small deposits will pay five times as much as someone you deposits the same amount in one transaction and someone who withdraws often isn't paying for it.
As for other sites not in the gambling space localbitcoins starting doing the same a while back.